It is has already been proposed to design the valve drive train in an internal combustion having multiple valves to allow the primary and secondary valves to have different events. An example of a camshaft which allows two inlet valves to have different events is shown EP-A-0 319 956. It has furthermore been suggested to vary the phase of the secondary valve in relation to the primary valve in dependence upon engine operating conditions.
In the prior art, the event timing of each valve has been selected or controlled with a view to determining the charging of the engine, the aim often being to improve the volumetric efficiency by maximising the mass of the intake charge. As a result, in the prior art, where a primary valve and a secondary valve have different events, the difference has been a major one affecting either the durations or the relative phasing of the events.
The present invention is not, by contrast, concerned with controlling the charging of the engine cylinders and relates to engines in which the primary and secondary valves have substantially the same event timing and duration, the purpose of providing two valves being only to increase the combined valve skirt area and the through flow cross section of the valves when they are open.
The invention is instead concerned with the noise emitted from an engine. In any engine, the valve mechanism is a source of noise and this problem is accentuated in a multi-valve engine since there are more valves to operate.